The present invention relates to a photolithography to transfer a microfine pattern onto a specimen substrate to form semiconductor integrated circuits and liquid crystal display panels, and more particularly to an exposure method, an aligner and a method of manufacturing semiconductor integrated circuits, which use a mask to control the phase of light for exposure.
Pattern forming for the semiconductor integrated circuits and liquid crystal display panels commonly uses a lithography technology, a method of transferring a pattern formed on a mask onto a substrate by exposure. This pattern transfer is generally performed by a reduction projection type projection aligner that reduces and transfers the pattern formed on the mask.
As the patterns have become increasingly fine in recent years, the design rules of circuit devices and wiring have been decreasing rapidly, for example, down to less than 0.5 .mu.m. The projection aligner is thus required to have a higher resolution than ever.
When, as a result of enhanced integration of semiconductor integrated circuits, the design rules of circuit devices and wiring are of the order of submicrons, however, the photolithography process, that uses light such as g and i beams to transfer a semiconductor integrated circuit pattern onto the semiconductor wafer, poses a serious problem of deteriorated precision of circuit pattern transfer.
Consider a case where a light-shielding area is adjoined by light-transmissive areas on both sides thereof. Because the beams of light that pass these light-transmissive areas are in phase, these light beams that have passed through the light-transmissive areas interfere with and intensify each other at the transfer position of the light-shielding area, with the result that the contrast of a projected image on the semiconductor wafer decreases and the focus depth becomes shallow. This in turn reduces the pattern transfer precision significantly, making the transfer of microfine circuit patterns difficult.
To cope with this situation, a technology is available which increases the numerical aperture (NA) of an optical system of an aligner while leaving the wavelength of light as is to improve the resolution. Generally, the greater the numerical aperture of the projection lens or the shorter the wavelength of light used for exposure, the better the resolving power. The method of increasing the NA, however, gives rise to a problem of lowering the focus depth during pattern transfer.
Efforts have been made to shorten the wavelength of exposure light, from g and i beams generated from a mercury lamp to an excimer laser beam. Shortening the wavelength further below the excimer laser, however, poses a problem of raising various restrictions in terms of light source, optical system material or resist material.
Because of these restrictions, a variety of exposure technologies are being studied which use the existing projection aligner and improve the resolution while securing the focus depth. One such representative means currently proposed is a phase shift technology that manipulates the phase of light passing through the mask in the exposure process to improve the contrast of a projected image.
Japan Patent Publication No. 59296/1987, for example, discloses a phase shift technology, in which a transparent film is provided at one of paired light-transmissive areas adjoining a light-shielding area on both sides to cause a phase difference between beams of light that have passed through the two light-transmissive areas during the exposure process so that the interfering beams weaken each other at a light-shielding area on the semiconductor wafer.
Another example is Japan Patent Publication No. 50811/1987, which discloses a method in which a transparent member (phase shifter) for inverting the phase of light is provided at a particular light-transmissive area on the mask to utilize interference between beams of light. This method makes significant improvements on the resolution, particularly, for patterns that have a periodicity.
Still another method is Japan Patent Laid-Open No. 67514/1987, which discloses a method in which an auxiliary pattern, fine openings, is provided to the periphery of an isolated, fine main pattern formed in the light-shielding area of the mask to produce a phase difference between these two patterns. This mask prevents expansion of the main pattern image by the presence of the auxiliary pattern to improve the resolution of the isolated pattern.
Further, Japan Patent Laid-Open No. 140743/1990 discloses a phase shift technology, in which a phase shifter is provided at a part of the light-transmissive area of the mask to generate a phase difference between the transmitting beams and thereby emphasize the boundary of the phase shifter.
Furthermore, Japan Patent Laid-Open No. 270213/1991 discloses a method, in which two beams of light that have passed through different parts of a mask formed with a pattern of light-transmissive and light-shielding areas are made to be in reverse phase with each other immediately after they passed through the mask and in which these two beams are then synthesized and irradiated against the specimen to improve the contrast of the projected image.